The present invention relates to an antenna duplexer for use in a radio communication apparatus.
Generally, a radio communication apparatus requires an antenna duplexer in order to use a single antenna commonly for transmission and reception. Various constructions of antenna duplexers are now available, depending on the type of circuit arrangement of a transceiver. It is well known to use a cascade circuit having local oscillation filter, reception filter and transmission filter when a received signal is directly applied to a first mixer. The pass bands of the local oscillation, reception and transmission filters are respectively set at the first local oscillator frequency band, reception frequency band and transmission frequency band.
It has been customary to construct such an antenna duplexer by a use of helical filters, dielectric filters, planar line filters or the like. An example of antenna duplexers using dielectric filters is disclosed in Fukusawa's "Miniaturized Microwave Filter Construction with Dielectric-Loaded Resonator and Space Coupling," IEEE MTT-S I.M.S. Digest, pp 209-211, June 16, 1981.
In parallel with the remarkable progress in the production of compact and more economical radio communication apparatuses, there is an increasing demand for reductions both in size and in cost of antenna duplexers, as well as for various circuit elements such as semiconductors. The helical filters, dielectric filters, planar line filters or the like occupy a substantial volume at the present stage of development, although efforts have been made to provide a more compact design. Additionally, such filters require some degrees of adjustment to satisfy expected characteristics. Concerning a more compact and economical antenna duplexer, the above-mentioned traditional filter arrangement has almost lost its prospect and is one of stumbling blocks for further improvements.
Meanwhile, SAW (surface acoustic wave) devices are also known as filter elements and are now rapidly progressing toward practical use. The use of SAW filters is now rapidly increasing in practical applications. Moreover, because a SAW filter is manufactured by electron beam exposure, photoetching, dry etching or a similar technique, any inaccuracy at the production stage is almost too small to require adjustments only if due consideration is given to design conditions for attaining desired characteristics.
However, SAW filters still involve the following problem when applied to antenna duplexers. A commonly-used and small-sized radio communication apparatus usually generates about several hundreds mW to several W output power. In such high power operation, the SAW filter is likely to be burned by heat generation which occurs due to an insertion loss of the transmission filter and by a low voltage immunity to a transmission signal which the reception filter reflects outside its pass band. This is due to the fact that the SAW filter inevitably requires a minute electrode pattern. In addition, it is difficult under the present situation to solve the problem concerned with relying on further improvements in performance or on similar measures.